Nickel Titanium (NiTi) alloys are known in the prior art. They display excellent shape memory behavior and possess outstanding specific work output, along with other unusual properties such as superelastic behavior as disclosed in K. Otsuka and X. Ren, Prog. Maters, Sci. 50 (2005) 511, good damping behavior, and erosion and wear resistance. However, their practical usage range does not exceed approximately 60° C., which is the maximum martensite start temperature (Ms) observed in binary alloys. They are often marketed as “Nitinol”, and are used as connectors, couplings, valves, seals, coatings, actuators, and in various medical and dental devices. Their Ni/Ti content ranges from approximately 48.5/51.5 atom % to approximately 51/49 atom %.
The unique behavior of NiTi and related shape memory alloys is based on the reversible temperature-dependent austenite-to-martensite phase transformation. They can easily be deformed from an initial shape to another while in their lower temperature form (martensite). Upon heating through the phase transformation to austenite, they revert back to their original shape. This reversible behavior is the basis for such properties as shape memory effect and superelasticity and is the foundation for many practical applications. In addition, a subset of the alloys that display shape memory behavior can recover their original shape even when a substantial opposing force is applied to the material. In this way, the alloy is capable of performing work and can act as a solid state actuator. NiTi, in particular, is capable of specific work output with equivalent or higher energy density than pneumatic actuators or D.C. motors, see C. Mavroidis, res Nondestr. Eval. 14 (2002) 1.
The reversible transformation of Ni—Ti shape memory alloys occurs over two slightly different temperature ranges: one range during heating and the other during cooling. These temperature ranges are characteristic of the specific alloy and dependent on alloy composition and processing history. When shape memory alloys are heated they will change from their martensite form to austenite. The temperature at which this transformation starts is the austenite start temperature (As) and the temperature at which this transformation from martensite to austenite is complete is called the austenite finish temperature (Af). When the alloy is cooled from high temperature, the transformation is reversed and the austenite will begin transforming to martensite at the martensite start temperature (Ms) and will be completely converted to martensite at the martensite finish temperature (Mf). Therefore, these transformation temperatures determine over what temperature range the shape memory and other effects can be observed. For NiTi, the maximum Ms temperature is observed in stoichiometric Ni-50 at, % Ti and Ti-rich NiTi alloys and is about 60° C., as disclosed in Otsuka and Ren, Prog. Mater. Sci. 50 (2005) 511.
The temperature range for the martensite-to-austenite transformation that takes place on heating is somewhat higher than that for the reverse transformation on cooling. The difference between the transition temperatures on heating and cooling is referred to as the hysteresis and is usually defined as the difference between the Af and Ms temperatures. For NiTi alloys, this difference is on the order of 20° C.-30° C. (Otsuka and Ren, Prog. Mater. Sci. 50 (2005) 511). When developing shape memory alloys for actuator applications where active control of the material is desired, or for applications involving a quick response time, the hysteresis should be as narrow as possible.
Thus, while the unique behavior of NiTi is widely exploited in applications near room temperature, the uses for a material with properties similar to Ni—Ti, but capable of:
1. being used at elevated temperatures (greater than 100° C.),
2. having low hysteresis, and
3. high specific work output
would open up new applications for shape memory alloys, particularly in the aerospace, automotive, automation and controls, appliance, energy, chemical processing, heating and ventilation, safety and security, and electronics (MEMS devices) industries.
Alloying additions to NiTi are well known and are primarily used to affect changes in the transformation temperatures of the resulting material: either to increase or decrease the transformation temperatures or change the width of the hysteresis. For example, Fe or Co substituted for Ni; or Al, Mn, V, or Cr substituted for Ti will severely decrease the transformation temperatures of the resulting ternary alloys. Cu has relatively little effect on the transformation temperatures but it can significantly reduce the hysteresis. Nb, on the other hand, broadens the hysteresis. There is also a group of alloying additions (Pd, Pt, Au, Hf, and Zr) that can raise the transformation temperatures of NiTi. Current proposed alloying schemes involve either substitution of a precious metal for Ni, e.g., (Ni,Pd)Ti, (Ni,Pt)Ti, (Ni,Au)Ti as described by Lindquist, in “Structure and Transformation Behavior of Martensitic Ti—(Ni,Pd) and Ti—(Ni,Pt) Alloys”, Thesis, University of Illinois, 1978 and Wu, “Interstitial Ordering and Martensitic Transformation of Titanium-Nickel-Gold Alloys”, Thesis, University of Illinois 1986), or substitution of a reactive element like Hf or Zr for Ti, e.g., Ni(Ti,Hf) and Ni(Ti,Zr), (as claimed by AbuJudom, II et al., U.S. Pat. No. 5,114,504). However, the available literature does not report data for an alloy that exhibits a high transformation temperature with narrow hysteresis and demonstrated high work output.
Ni—Pt—Ti (20/30/50 atom %) alloys (R. Noebe, et al., SPIE Conf. Proc. Vol. 5761, (2005), pp. 364-375) have a high transition temperature but do not have sufficient work output to be used as acceptable actuators. Of the high-temperature ternary NiTi-based systems, useful work characteristics have only been verified in Ni(Ti,Hf) thin films by Rasmussen et al. (U.S. Pat. No. 6,454,913 and Patent Pub. No. US2002/01 8971 9A1). However, these Ni(Ti,Hf) alloys can only be used in applications requiring repeated actuation at temperatures up to about 100° C. because their wide hysteresis results in an Mf near or below 100° C. There is little chance of increasing their use temperature (transformation temperature) by further increasing the Hf level due to microstructural limitations. The wide hysteresis of the Ni(Ti,Hf) alloys also makes them unsuitable for applications where active control is involved. To our knowledge, work measurements have not been made public on any of the other potential high-temperature NiTi-based systems.
Methods for processing high temperature shape memory alloys (HTSMA) have also been described in the literature. Tuominen, et al. (U.S. Pat. No. 4,865,663 claimed a series of (Ni,Pd)Ti—B alloys for increased fabricability. Goldberg et al. (U.S. Pat. No. 5,641,364) have claimed a method for manufacturing and heat treating (Ni,Pd)Ti, Ni(Ti,Zr) and Ni(Ti,Hf) alloys for improved shape recovery characteristics. A patent has also been applied for the method of producing (Ni,Pd)Ti thin films by magnetron sputtering (Rasmussen et al., US patent Pub. No. 2003/0168334). None of these references discuss shape recovery for alloys under load or provide any other indication of the work output for any of the materials discussed.
Lindquist (“Structure and Transformation Behavior of Martensitic Ti—(Ni,Pd) and Ti—(Ni,Pt) Alloys,” Thesis, University of Illinois, 1978) reported the effect of Pt additions on the transformation temperature of “stoichiometric alloys” of (Ni+Pt):Ti. These alloys contain exactly 50 atom percent Ti. Lindquist studied Pt contents ranging from 0 to 30 atom % and demonstrated a significant increase in transformation temperatures when the Pt content was between 10 and 30 atom %. Lindquist found that alloys with 20 atom % Pt were capable of unconstrained shape memory behavior but made no attempt to measure the shape recovery of the material against any biasing force so no indication of the potential work output for the material was determined. Johnson et al. (U.S. Pat. No. 6,669,795) described a method for making HTSMA thin films using multiple sputtering targets where the goal was to create an alloy that has a 1:1 atomic ratio of (Ni+X):Ti or Ni:(Ti+Y) where X is an element from the right side of the periodic table such as Pd, Pt, Au, or Cu and Y is an element from the left side of the periodic table such as Hf or Zr. However, no alloys were actually produced nor were any properties reported.
Thus, a viable shape memory alloy for high temperature applications, particularly for actuator-related functions, has yet to be developed.